Running and carbs. eek.

I've been tricked, last-minute, into running a half marathon this Saturday. I ran a couple this spring but now I'm down to running about 15-20 miles a week. I'm not worried about my time (I can let myself run super slowly, if need be), but I am slightly concerned about my carb intake. I have been on this diet for about 6 weeks and I think my body's pretty used to tough workouts while not eating carbs. I was thinking that the night before I would have a lot of veggies (with some protein) and some yogurt/milk (some of the higher-carb dairy), and that in the morning I would allow myself whole wheat toast and maybe some peanut butter or fruit or maybbbe a lower-carb energy bar? I assume that what I eat pre-run and even mid-run (GU packets) will be immediately burned up through running so I don't need to worry? Any thoughts?